Protective coating on positive lithium-metal-oxide electrodes for lithium batteries

ABSTRACT

A positive electrode for a non-aqueous lithium cell comprising a LiMn 2−x M x O 4  spinel structure in which M is one or more metal cations with an atomic number less than 52, such that the average oxidation state of the manganese ions is equal to or greater than 3.5, and in which 0≦x≦0.15, having one or more lithium spine oxide LiM′ 2 O 4  or lithiated spinel oxide Li 1+y M′ 2 O 4  compounds on the surface thereof in which M′ are cobalt cations and in which 0≦y≦1.

RELATED APPLICATIONS

This application, pursuant to 37 C.F.R. § 1.78(c), claims priority basedon provisional application Ser. No. 60/352,899 filed Jan. 29, 2002.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy (DOE)and The University of Chicago representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to positive lithium metal oxide electrodes forlithium cells and batteries, preferably rechargeable lithium-ion cellsand batteries. More specifically, it relates to a lithium metal oxideelectrode with a spinel-related structure with a surface coating of oneor more other lithium-containing spinel oxides that are structurallycompatible with, but compositionally different from the structure of thebulk lithium metal oxide electrode, to protect the bulk electrode fromcapacity loss effects, such as oxygen loss and manganese dissolutionduring the electrochemical cycling of lithium-ion cells.

BACKGROUND OF THE INVENTION

State-of-the-art lithium-ion cells have a lithiated carbon negativeelectrode, or anode, (Li_(x)C₆) and a lithium-cobalt-oxide positiveelectrode, or cathode, Li_(1−x)CoO₂. During charge and discharge of thecells lithium ions are transported between the two host structures ofthe anode and cathode with the simultaneous oxidation or reduction ofthe host electrodes, respectively. When graphite is used as the anode,the voltage of the cell is approximately 4 V. The cathode materialLiCoO₂, which has a layered structure, is expensive and becomes unstableat low lithium content, i.e., when cells reach an overcharged state atx≧0.5. Alternative less expensive electrode materials that areisostructural with LiCoO₂, such as LiNi_(0.8)Co_(0.2)O₂ andLiNi_(0.5)Mn_(0.5)O₂ are being developed in the hope of replacing atleast part of the cobalt component of the electrode. However, all theselayered structures, when extensively delithiated are unstable, becauseof the high oxygen activity at the surface of the particles; therefore,the electrode particles tend to react with the organic solvents of theelectrolyte or lose oxygen.

Spinel electrodes, such as those in the manganese-based systemLi_(1+x)Mn_(2−x)O₄, are particularly attractive alternatives to LiCoO₂because, not only are they relatively inexpensive, but they arethermally more stable than Li_(1−x)CoO₂ or Li_(1−x)Ni_(0.8)Co_(0.2)O₂ atlow lithium loadings, and because they do not contribute to theimpedance rise of electrochemically cycled lithium-ion cells to the sameextent as Li_(1−x)CoO₂ or Li_(1−x)Ni_(0.8)Co_(0.2)O₂ electrodes.

The Li_(1−x)[Mn₂]O₄ spinel system has been investigated extensively inthe past as an electrode for lithium-ion batteries. A major reason whythe spinel system has not yet been fully commercialized is because theelectrode is unstable in the cell environment, particularly if theoperating temperature of the cells is raised above room temperature, forexample, to 40–60° C. It is now generally acknowledged that thesolubility of Li_(x)[Mn₂]O₄ electrodes in acid medium occurs by thedisproportionation reactionMn³⁺ _((solid))→Mn⁴⁺ _((solid))+Mn²⁺ _((solution))  (1)during which the Mn²⁺ ions go into solution, and the Mn⁴⁺ ions remain inthe solid spinel phase. Such a reaction can occur in lithium-ion cellsbecause the hydrolysis of fluorinated lithium salts such as LiPF₆ withsmall amounts of residual water in the organic-based electrolytesolvents can generate hydrofluoric acid, HF.

Full electrochemical delithiation of Li[Mn₂]O₄ leaves λ-MnO₂ with the[Mn₂]O₄ spinel framework. Like many manganese dioxides, λ-MnO₂ is apowerful oxidizing agent and can be readily reduced. Therefore, anyoxygen that may be evolved at the particle surface of the spinelelectrode at the top of charge will result in Mn³⁺ ions at the electrodesurface; the instability of Mn³⁺ ions at the high potential of thecharged cell will also drive the disproportionation reaction (1) shownabove, thus damaging the spinel surface and resulting in someirreversible capacity loss of the cell.

The presence of tetragonal Li₂[Mn₂]O₄ has also been observed in verysmall amounts at the surface of Li[Mn₂]O₄ spinel electrodes at the endof discharge after high rate cycling (C/3 rate) between 4.2 and 3.3 Vvs. Li. The compound Li₂[Mn₂]O₄ in which all the manganese ions aretrivalent will be unstable, like Li[Mn₂]O₄, at high potentials in a 1 MLiPF₆/EC/DMC electrolyte that contains HF, particularly if the lithiumcells are operated at 40–50° C. In this case, a disproportionationreaction occurs in which MnO dissolves from the particle surface toleave an insoluble and stable Li₂MnO₃ rock-salt phase. This reaction mayaccount for some of the capacity loss of 4-V Li/Li_(x)[Mn₂]O₄ cells onlong-term cycling.

Substantial efforts have already been made in the past to overcome thesolubility problems associated with the Li[Mn₂]O₄ spinel electrode. Forexample, partial substitution of the manganese ions in Li[Mn₂]O₄ with amono-, di- or trivalent ion changes the composition of the electrode andincreases the average oxidation state of the manganese ions above 3.5,thus reducing the amount of Mn³⁺ ions in the fully discharged electrode.Other approaches to suppress manganese dissolution from the spinelelectrode have been taken, for example, by protecting the spinelparticles with a surface coating, such as a low-melting lithium borateglass or a coating of LiCoO₂ applied at high temperature (e.g., 700–800°C.) both of which are known to be more resistant to dissolution in theelectrolytes than Li[Mn₂]O₄. Alternatively, a coating of ZrO₂ or Co₃O₄has been applied to the electrode particles. Although some success hasbeen achieved by using these approaches, the problems of electrodeinstability have not yet been fully resolved and further improvementsare necessary.

LiMn₂O₄ spinel electrodes have a tendency to lose oxygen or react withthe electrolyte if charged to a high potential, such as 4.5 V, whichcauses irreversible capacity loss effects. Moreover, the loss of oxygenfrom the electrode can also contribute to exothermic reactions with theelectrolyte and with the lithiated carbon negative electrode, andsubsequently to thermal runaway if the temperature of the cell reaches acritical value. There is therefore a strong requirement to improve thestate-of-the-art protective coatings on these electrodes to improve theoverall performance and safety of lithium-ion cells.

SUMMARY OF THE INVENTION

This invention relates to an improved LiMn_(2−x)M_(x)O₄ positiveelectrode (0≦x≦0.15) with a spinel-related structure for non-aqueouslithium cells and batteries, preferably rechargeable lithium-ion cellsand batteries. More specifically, it relates to a LiMn_(2−x)M_(x)O₄spinel electrode with a surface coating of one or more other lithiumspinel oxides LiM′₂O₄ or lithiated LiM′₂O₄ spinel oxides Li_(1+y)M′₂O₄(0<y≦1) that are structurally compatible with, but compositionallydifferent from the structure of the bulk LiMn_(2−x)M_(x)O₄ spinelelectrode, to protect the spinel electrode in the bulk from capacityloss effects, such as oxygen loss and/or manganese dissolution duringthe electrochemical cycling of lithium-ion cells. The bulkLiMn_(2−x)M_(x)O₄ spinel electrode is comprised of the family ofcation-stabilized spinels LiMn_(2−x)M_(x)O₄, in which M is one or moreof any mono or multivalent cations with an atomic number less than 52,preferably with an atomic number less than 33, such as monovalent Li⁺ orH⁺, divalent Mg²⁺ or Co²⁺, trivalent Al³⁺ or Co³⁺, tetravalent Ti⁴⁺ orZr⁴⁺, or the like, such that the average oxidation state of themanganese ions is equal to or greater than 3.5. The lithium spinel oxideor lithiated spinel oxide coatings are comprised of LiM′₂O₄ orLi_(1+y)M′₂O₄ compounds in which the M′ cations are selected from one ormore of lithium, cobalt, titanium or manganese, preferably lithiumand/or cobalt, for example, a lithium-cobalt-oxide spinelLi_(x)Co_(3−x)O₄ (0<x<0.4), or alternatively, the low-temperature,lithiated-spinel form of LiCoO₂ (i.e., Li₂Co₂O₄) in which the CoO₂component has a [Co₂]O₄ spinel-like framework. Other examples includethe lithium spinel oxides Li[Ti_(1.67)Li_(0.33)]O₄ orLi[Mn_(1.67)Li_(0.33)]O₄, or their electronically-conducting Li-, Mg- orAl-substituted derivatives to induce mixed valence character to the Tiand Mn transition metal cations and hence electronic conductivity to thespinel coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

FIG. 1 depicts the powder X-ray diffraction pattern of aLi_(1.03)Mn_(1.97)O₄ electrode coated with a lithium-cobalt-oxidespinel, the coating prepared by a sol-gel method from lithium acetateand cobalt acetate precursors at 400° C.;

FIG. 2 depicts a transmission electron micrograph of the surface of aLi_(1.03)Mn_(1.97)O₄ electrode coated with a lithium-cobalt-oxidespinel, the X-ray diffraction pattern of which is shown in FIG. 1;

FIG. 3 depicts the powder X-ray diffraction pattern of aLi_(1.03)Mn_(1.97)O₄ electrode coated with a lithium-cobalt-oxidespinel, the coating prepared from octacarbonyidicobalt with 1–5 molepercent hexane stabilizer and lithium carbonate precursors at 400° C.;

FIG. 4 depicts the electrochemical profile of a standardLi/Li_(1.03)Mn_(1.97)O₄ cell and a Li/Li_(1.03)Mn_(1.97)O₄ cell in whichthe Li_(1.03)Mn_(1.97)O₄ electrode was coated with LiCoO₂ by a sol-gelmethod at 400° C.;

FIG. 5( a)–(c) depict plots of electrode capacity vs. cycle number of a)a standard Li/Li_(1.03)Mn_(1.97)O₄ cell and b) and c)Li/Li_(1.03)Mn_(1.97)O₄ cells in which the Li_(1.03)Mn_(1.97)O₄electrode was coated with a lithium-cobalt-oxide spinel;

FIG. 6 depicts a schematic illustration of an electrochemical cell; and

FIG. 7 depicts a schematic illustration of an example of a batteryemploying the cells of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention relates to an improved LiMn_(2−x)M_(x)O₄ spinel electrode(0≦x≦0.15), in which M is one or more of any mono or multivalent cationswith an atomic number less than 52, preferably with an atomic numberless than 33, such as monovalent Li⁺ or H⁺, divalent Mg²⁺ or Co²⁺,trivalent Al³⁺ or Co³⁺, tetravalent Ti^(4+ or Zr) ⁴⁺, or the like, suchthat the average oxidation state of the manganese ions is equal to, orgreater than 3.5 for lithium cells and batteries, preferablyrechargeable lithium-ion cells and batteries. More specifically, itrelates to a LiMn_(2−x)M_(x)O₄ spinel electrode with a surface coatingof one or more other lithium spinel oxides LiM′₂O₄ or lithiated LiM′₂O₄spinel oxides Li_(1+y)M′₂O₄ (0<y≦1) that are structurally compatiblewith, but compositionally different to the structure of the bulkLiMn_(2−x)M_(x)O₄ spinel electrode, to protect the bulk spinel electrodefrom capacity loss effects, such as oxygen loss and/or manganesedissolution during the electrochemical cycling of lithium-ion cells.

Of particular significance to this invention is the realization that inorder to achieve good binding between a surface protective coatingconsisting of one or more lithium-metal oxides with the lithium metaloxide structure of the bulk electrode, it is important to have a strongstructural compatibility between the surface coating and the structureof the bulk electrode. Therefore, according to the invention, both thelithium metal oxide of the surface coating and the bulk electrode havespinel structures which are close-packed, preferably cubic-close-packedor approximately cubic-close-packed, with compatible crystallographicparameters that allow the structure of the coating to be fused to, orintergrown with, or connected to the structure of the bulk of theparticle at the coating/bulk interface.

Attempts have been made in the past to coat Li[Mn₂]O₄ spinel particleswith LiCoO₂ at high temperature, typically 800° C. At this temperature,LiCoO₂ has a layered structure with trigonal symmetry (R-3m, a=2.82 Å,c=14.06 Å), whereas Li[Mn₂]O₄ has a spinel structure with cubic symmetry(Fd-3m, a=8.24 Å). There is thus an incompatibility in structure typesbetween the LiCoO₂ coating and the spinel structure of the bulkelectrode, which may affect 1) the integrity of the surface layer,particularly at the surface/bulk interface during cycling and 2) itsadhesion to the spinel particles. Stabilizing Li[Mn₂]O₄ spinelelectrodes can be achieved with other spinel compounds with cubic Fd-3msymmetry or close to cubic symmetry, with lattice parameters close tothat of the Li[Mn₂]O₄ spinel electrode. In this respect, there are anumber of suitable candidate materials with a spinel structure areuseful as a protective coating. One such compound is the lithiatedspinel Li₂[Co₂]O₄ which, although having the same formula as layeredLiCoO₂, has a different structure; Li₂[Co₂]O₄ can be synthesized at alower temperature than layered LiCoO₂, typically at 400° C. or lower.For example, the lattice parameter of a Li_(1−x)[Mn₂]O₄ electrodechanges from 8.24 to 8.03 Å for the range 0<x<1. The lithiated spinelLi₂[Co₂]O₄ has a lattice parameter of approximately the same dimension(8.00 Å), and this parameter does not change significantly on lithiumextraction to the stoichiometric spinel composition Li[Co₂]O₄. Thus,according to the invention, the cobalt spinel has suitable structuralcharacteristics for the protective layer. Furthermore, it is known thatLiCoO₂ materials can be fabricated at moderate temperatures (e.g.,between 400 and 500° C. with a cation distribution which is intermediatebetween that of layered-LiCoO₂ and lithiated-spinel LiCoO₂ (Li₂[Co₂]O₄).The applicants believe that such intergrown materials withspinel-related character provide greater stability than the structureswith the ideal spinel arrangement of cations. The invention alsoincludes lithium-substituted cobalt oxide spinels Li_(x)Co_(3−x)O₄ thathave been reported to exist over the range 0<x<0.4 by N. K.Appandairajan et al in the Journal of Power Sources, Volume 40, pages117–121 (1981), as the protective coating. The protective layers orcoatings need not be uniform or homogeneous; indeed it has been foundthat the layers or coatings can be comprised of individual orinterconnected grains that are fused to the surface of the bulk spinelparticles. Therefore in a preferred embodiment, the invention includeslithium spinel oxides LiM′O₄ or lithiated LiM₂O₄ spinel oxidesLi_(1+y)M′₂O₄ (0<y≦1) in which M′ can be selected from lithium and/orcobalt on the surface of the LiMn_(2−x) M_(x)O₄ electrode.

It is known in the art of lithium battery technology that LiMn₂O₄ spinelelectrodes can be stabilized by substituting Co²⁺ or Co³⁺ ions for theMn^(4+/3+) ions in the bulk of the spinel structure, thereby reducingthe concentration of Mn³⁺ ions in the spinel framework and increasingthe stability of the spinel electrode. Such Co-stabilized spinels can berepresented as Li[Mn_(2−x)Co_(x)]O₄ in which x can be typically 0.15 orless, preferably 0.05 or less. Therefore, in a yet a further embodimentof this invention, these stabilized spinels can be used to good effectby introducing a concentration gradient of cobalt in the spinelframework, which increases from a low concentration at the center of theelectrode particle (e.g., with composition [Mn_(1.85)Co_(0.15)]O₄) to ahigh concentration at the surface (e.g., [Co₂]O₄). The gradual change inlattice parameter of the spinel electrode associated with change in Coconcentration, it is believed will contribute to the structuralstability of the electrode and to enhanced stability at the electrodesurface.

The principles of this invention as described above can be extended toinclude other lithium spinel materials or lithiated spinels as theprotective layer, for example, LiM′₂O₄ or Li_(1+y)M′₂O₄ spinel-relatedcompounds in which M′ is selected from one or more cations of lithium,titanium and manganese, such as the lithium spinel oxidesLi[Ti_(1.67)Li_(0.33)]O₄ or Li[Mn_(1.67)Li_(0.33)]O₄, that have latticeparameters of a=8.36 Å and a=8.14 Å, respectively, close to the typicallattice parameters of the LiMn_(2−x)M_(x)O₄ bulk spinel electrodes ofthis invention (≦8.24 Å). It has already been well documented in theliterature that many spinel oxides with a wide variety of compositionscan be synthesized in the laboratory, such as those having cations withan atomic number less than 52 as reported by R. J. Hill et al in Phys.Chem. Minerals, Volume 4, pages 317 to 339 (1979). It is also well knownin the art that lithium ions can be ion-exchanged with protons (H⁺ ions)from typical electrolytes of lithium cells, thereby leading to theincorporation of the H⁺ ions within the spinel electrode structure onstorage and during the electrochemical operation of cells. In aparticular embodiment, the invention includes Li-, Mg- or Al-substitutedderivatives of the lithium spinel oxides LiM′₂O₄ or Li_(1+y)M′₂O₄ suchas Li[Ti_(1.67)Li_(0.33)]O₄ or Li[Mn_(1.67)Li_(0.33)]O₄ as reported byC. H. Chen et al in the Journal of the Electrochemical Society, Volume148(1), pages A102 to A103 (2001) to induce mixed valence character tothe Ti and Mn transition metal cations and hence electronic conductivityto the spinel coating, examples for the lithium titanate spinel coatingbeing Li[Ti_(1.67+z)Li_(0.33−z)]O₄, Li[Ti_(1.67)Li_(0.33−z)Mg_(z)]O₄ andLi[Ti_(1.67)Li_(0.33−z)Al_(z]O) ₄ for the lithium manganate spinelcoating being Li[Mn_(1.67)Li_(0.33−z)Mg_(z)]O₄ andLi[Mn_(1.67)Li_(0.33−z)Al_(z)]O₄ for 0<z<0.2. A particular advantage ofusing LiMn_(2−x)M_(x)O₄, LiM′₂O₄ or Li_(1+y)M′₂O₄ lithium spinel oxidesthat contain some aluminum ions, titanium ions or zirconium ions is thatthese ions bond strongly to the oxygen framework of the spinel, thusproviding additional structural stability to the electrode.

The following examples describe possible methods of synthesizing thecoated lithium-metal oxide electrodes of this invention as contemplatedby the inventors, but they are not to be construed as limiting examples.

EXAMPLE 1

A sample of Li_(1.03)Mn_(1.97)O₄ spinel powder was suspended inmethanol, to which was added a 1:1 molar ratio of lithium acetate andcobalt acetate, such that the LiCoO₂ content in the final electrode was3 percent by weight. The mixture was thoroughly stirred. The methanolwas removed by rotary evaporation. The resulting product was heattreated under flowing oxygen at 400° C. for 96 hours. The X-raydiffraction pattern of the lithium-cobalt-oxide-coatedLi_(1.03)Mn_(1.97)O₄ sample is compared with the parent, uncoatedLi_(1.03)Mn_(1.97)O₄ starting material, as shown in FIG. 1. FIG. 1demonstrates that the X-ray diffraction pattern of the coated sample iseffectively identical to that of the parent uncoated spinel material. Atransmission electron microscope image of the Li—Co—O spinel coating ona Li—Mn—O spinel particle is shown in FIG. 2. An EDAX (ElectronDispersion Analysis of X-rays) analysis of the coating showedunequivocally that the surface coating contained cobalt and that thebulk of the electrode particle contained manganese. However, theapplicants believe that diffusion of lithium, cobalt and manganese takesplace within the close-packed oxygen array and at the phase boundariesduring the coating process at 400° C. Therefore, concentration gradientsat the coating/bulk interface of the electrodes. FIG. 2 clearly shows inbroad outline the distinguishing boundary that connects the Li—Co—Ospinel coating and the Li—Mn—O spinel in the bulk of the particle. FIG.2 also demonstrates that the coating, when applied by the sol-geltechnique, is not homogeneous or uniform, and that it is comprised ofindividual, but interconnected grains of Li—Co—O spinel that containplanar defects, the directions of which are indicated by the dottedarrows in FIG. 2.

EXAMPLE 2

A sample of Li_(1.03)Mn_(1.97)O₄ spinel powder was suspended in hexanewith rapid stirring. A predetermined quantity of octacarbonyldicobaltwith 1–5 mole percent hexane as stabilizer, designated Co₂(CO)₈.C₆H₁₄,and Li₂CO₃ (Li:Co ratio=1:1) was used to fabricate alithium-cobalt-oxide spinel coating such that the overall composition ofthe spinel electrode was 0.2LiCoO₂.0.8Li_(1.03)Mn_(1.97)O₄. TheCo₂(CO)₈.C₆H₁₄ was dissolved in hexane and added to the LiMn₂O₄ sampletogether with the required quantity of Li₂CO₃. The temperature wasraised slowly to evaporate the hexane. The resulting product was thenheat treated under flowing oxygen at 400° C. for 96 hrs. The X-raydiffraction pattern of the coated Li_(1.03)Mn_(1.97)O₄ sample is shownin FIG. 3. This X-ray diffraction pattern provides evidence of thecoated Li_(1.03)Mn_(1.97)O₄ sample as well as a detectable amount of aspinel-related Li_(x)Co_(3−x)O₄ product, as evident from the weak [220]peak at approximately 31.5° 2θ and peak shoulders (arrowed) that arelocated to the right of the coated Li_(1.03)Mn_(1.97)O₄ peaks. Thedetection of a Li_(x)Co_(3−x)O₄ product in the electrode sample isattributed to the relatively high concentration of cobalt precursor usedin Example 2 compared with Example 1.

EXAMPLE 3

Standard spinel electrodes of approximate compositionLi_(1.03)Mn_(1.97)O₄, and those that had been coated with lithium cobaltoxides with spinel-related structures at a moderate temperature (400°C.), were evaluated in coin cells (size 2032, with dimensions 20 mmdiameter and 3.2 mm high) against a counter lithium electrode. The cellshad the configuration: Li/1M LiPF₆ in ethylene carbonate (EC), diethylcarbonate (DEC) (1:1)/Li_(1.03)Mn_(1.97)O₄. Electrodes were fabricatedwith approximately 7 to 10 mg of the spinel powder, i.e., approximately82% by weight of the laminate electrode, intimately mixed withapproximately 10% by weight of a polyvinylidene difluoride binder (Kynaror Kureha-type PVDF polymer) and approximately 8% by weight of carbon(graphite, such as Timcal SFG-6, or acetylene black, such as ChevronXC-72) in 1-methyl-2-pyrrolidinone (NMP). The slurries were coated witha doctor blade onto an aluminum foil substrate current collector. Theelectrode laminates were dried under vacuum at temperatures from 40 to110° C.; electrodes of suitable size were punched from these laminatesto fit the coin cells. Metallic lithium foil was used as the counterelectrode. The coin cells were discharged and charged at constantcurrent (typically 0.1 mA/cm²) within the voltage range 4.3 to 3.3 V.

The electrochemical voltage profiles, obtained on the fifth cyclebetween 4.3 and 3.3 V at 50° C., of a standard Li/Li_(1.03)Mn_(1.97)O₄cell and a Li/Li_(1.03)Mn_(1.97)O₄ cell in which the spinel electrodehad been coated by the sol-gel method described in Example 1 areprovided in FIG. 4. The voltage profile of the cell with a coated spinelelectrode shows a short plateau at approximately 3.6 V, which is absentin the profile of the standard Li/Li_(1.03)Mn_(1.97)O₄ cell; thisfeature is consistent with the electrochemical behavior of LiCoO₂ with aspinel-related structure Li₂[Co₂]O₄, synthesized at 400° C.

FIG. 5 shows plots of electrode discharge capacity versus cycle numberfor lithium cells that contained the standard, uncoatedLi_(1.03)Mn_(1.97)O₄ electrode and the coated electrodes of Examples 1and 2. The data in FIG. 5 indicate that at 50° C. the rate of capacityfade of the coated electrodes is significantly less than that of thestandard electrode, thereby demonstrating the effectiveness of thecoating. The theoretical capacity of the coated spinel electrode ofExample 1, taking into account the capacity of the electrochemicallyactive LiCoO₂ coating, is 140 mAh/g. After 25 cycles at 50° C., thiscoated electrode delivers a discharge capacity of approximately 111mAh/g (i.e., 79% of its theoretical capacity), in contrast to 103 mAh/gdelivered by the standard spinel electrodes, which is 76% of itstheoretical capacity (136 mAh/g). Of major significance, however, isthat the rate of capacity fade of the coated electrode for the first 25cycles is 0.14% per cycle, whereas for the standard electrodes it is0.50% per cycle.

Although FIG. 5 shows that the capacity of the coated electrode ofExample 2 is less than that of the standard, uncoatedLi_(1.03)Mn_(1.97)O₄ electrode, the inferior capacity of the coatedelectrode is attributed to the relatively high cobalt content used forthe coating that resulted in an appreciable amount of spinel-relatedLi_(x)Co_(3−x)O₄ in the electrode which does not contributesignificantly to the capacity of the cells when charged and dischargedbetween 4.3 and 3.3 V. Nevertheless, the rate of capacity fade of thecoated electrode of Example 2 is significantly superior to that of thestandard, uncoated Li_(1.03)Mn_(1.97)O₄ electrode. Moreover, the rate ofcapacity fade of the coated electrode of Example 2 is essentially thesame as the rate of capacity fade of the coated electrode of Example 1as reflected by the closely parallel slopes of their capacity vs. cyclenumber plots in FIG. 5. Therefore, the inventors believe that thepractical capacity of coated electrodes made in accordance with themethod of Example 2 will be increased by reducing the amount ofCo₂(CO)₈.C₆H₁₄ and Li₂CO₃ precursors used for making the Li—Co—Ocoating.

The examples and data provided above demonstrate the principles of thisinvention. In particular, they show that improved electrochemicalperformance of a non-aqueous lithium cell can be achieved by coating aLiMn₂O₄ spinel positive electrode with a lithium-containing spinelcompounds that are structurally compatible with, but compositionallydifferent from the structure of the bulk LiMn₂O₄ spinel electrode, toprovide a good lattice match between the bulk electrode structure andthe structure of the electrode coating, and to protect the spinelelectrode in the bulk from capacity loss effects, such as oxygen lossand/or manganese dissolution during the electrochemical cycling oflithium-ion cells.

This invention, therefore, relates to positive electrodes for anon-aqueous electrochemical lithium cell, as shown schematically in FIG.13, the cell represented by the numeral 10 having a negative electrode12 separated from a positive electrode 16 by an electrolyte 14, allcontained in an insulating housing 18 with suitable terminals (notshown) being provided in electronic contact with the negative electrode12 and the positive electrode 16. Binders and other materials normallyassociated with both the electrolyte and the negative and positiveelectrodes are well known in the art and are not described herein, butare included as is understood by those of ordinary skill in this art.FIG. 14 shows a schematic illustration of one example of a battery inwhich two strings of electrochemical lithium cells, described above, arearranged in parallel, each string comprising three cells arranged inseries.

While there has been disclosed what is considered to be the preferredembodiments of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention and thatadditional improvements in the capacity and stability of the electrodescan be expected to be made in the future by improving and optimizing theprocessing techniques whereby electrodes are coated with protectivelayers.

1. A positive electrode for a non-aqueous lithium cell comprising aLiMn_(2−x)M_(x)O₄ spinel structure in which M is one or more cationswith an atomic number less than 52, such that the average oxidationstate of the manganese ions is equal to or greater than 3.5, and inwhich 0≦x≦0.15, having one or more lithium spinel oxide LiM′₂O₄ orlithiated spinel oxide Li_(1+y)M′₂O₄ compounds on the surface thereof inwhich M′ are cobalt cations and in which 0≦y≦1.
 2. An electrodeaccording to claim 1 in which in which M is one or more metal cationswith an atomic number less than
 33. 3. An electrode according to claim 1in which M is selected from the group consisting of, Li⁺, Mg²⁺, Co²⁺,Al³⁺, Co³⁺, Ti⁴⁺ and Zr⁴⁺ cations.
 4. The positive electrode of claim 1,wherein the lithium spinel oxide or lithiated spinal oxide is present asa coating.
 5. The positive electrode of claim 1, wherein theconcentration of M′ cations increases from the center to the surface ofthe electrode.
 6. An electrode according to claim 1, in which M cationsare cobalt.
 7. An electrode according to claim 6, in which theLi_(1+y)M′₂O₄ compound is Li₂Co₂O₄.
 8. An electrode according to claim1, in which Li⁺ cations are partially substituted by H³⁰ cations.
 9. Thepositive electrode of claim 1, wherein the lithium spinel oxide LiM′₂O₄or lithiated spinel oxide Li_(1+y)M′₂O₄ compounds are present as acoating.
 10. The positive electrode of claim 1, wherein theconcentration of M′ cations increases from the center to the surface ofthe electrode.
 11. A positive electrode for a non-aqueous lithium cellcomprising a LiMn_(2−x)M_(x)O₄ spinel structure in which M is one ormore metal cations with an atomic number less than 52, such that theaverage oxidation state of the manganese ions is equal to or greaterthan 3.5, and in which 0≦x≦0.15, having a lithiated spinel compoundLi_(x)Co_(3−x)O₄ for 0<x<0.4 and y=0 on the surface thereof.
 12. Anon-aqueous lithium electrochemical cell comprising a negativeelectrode, an electrolyte and a positive electrode, the positiveelectrode comprising a LiMn_(2−x)M_(x)O₄ spinel structure in which M isone or more metal cations with an atomic number less than 52, such thatthe average oxidation state of the manganese ions is equal to or greaterthan 3.5, and in which 0≦x≦0.15, having one or more lithium spinel oxideLiM′₂O₄ or lithiated spinel oxide Li_(1+y)M′₂O₄ compounds on the surfacethereof in which M′ are cobalt cations and in which 0≦y≦1.
 13. Anon-aqueous lithium battery comprising a plurality of electrochemicalcells, electrically connected, each cell comprising a negativeelectrode, an electrolyte and a positive electrode, the positiveelectrode comprising a LiMn_(2−x)M_(x)O₄ spinel structure in which M isone or more metal cations with an atomic number less than 52, such thatthe average oxidation state of the manganese ions is equal to or greaterthan 3.5, and in which 0≦x≦0.15, having one or more lithium spinel oxideLiM′₂O₄ or lithiated spinel oxide Li_(1+y)M′₂O₄ compounds on the surfacethereof in which M′ are cobalt cations and in which 0≦y≦1.